Two decades after the importance of tryptophan catabolism for maintaining the immune privilege of the placenta was discovered (Munn, D. H. et al., Science, 1998, 281, 1191-1193), increasing evidence is extending its biological relevance beyond immune tolerance to non-self. According to the generally accepted concept, tryptophan, an essential amino acid, is catabolized in the local microenvironment of tumors, immune-privileged sites, or sites of inflammation (Mellor A L and Munn D H., Nat Rev Immunol, 2008, 8, 74-80). In these tissues, cancer cells, immune cells, or specialized epithelial cells (e.g., syncytiotrophoblasts in the placenta) create an immunosuppressive environment in tumors that shuts down antitumor immune responses in tumors and in tumor-draining lymph nodes by inducing T-cell anergy and apoptosis through depletion of tryptophan and accumulation of immunosuppressive tryptophan catabolites (Munn D H et al., J Exp Med., 1999, 189, 1363-1372; Fallarino F et al., Cell Death Differ., 2002, 9, 1069-1077).
It has recently been discovered that a key enzyme in tryptophan catabolism, tryptophan 2,3-dioxygenase (TDO2), which is considered responsible for regulating systemic tryptophan levels in the liver, is constitutively expressed in a wide variety of cancers, such as for example in bladder carcinoma, hepatocarcinoma, melanoma, mesothelioma, neuroblastoma, sarcoma, breast carcinoma, leukemia, renal cell carcinoma, colorectal carcinoma, head and neck carcinoma, lung carcinoma, brain tumor, glioblastoma, astrocytoma, myeloma, and pancreatic carcinoma (Pilotte L et al., Proc Natl Acad Sci USA, 2012, 109(7), 2497-502). TDO2 expression in tumor cells prevents tumor surveillance by the immune system and thus prevents tumor rejection by locally degrading tryptophan (Opitz C A et al., Nature, 2011, 478(7368), 197-203). The first evidence for this was provided through inhibition of TDO2 by a small molecule which inhibited tumor growth in a P815 mastocytoma tumor model with a prophylactic vaccination approach (Pilotte L et al., Proc Natl Acad Sci USA, 2012, 109(7), 2497-502). P815mTDO2 expressing tumors were rejected less in comparison to P815 tumors transfected with an empty vector, clearly demonstrating a growth benefit for TDO2 expressing tumors. Inhibition with a TDO2 inhibitor strongly decreased tumor growth in P815mTDO2 implanted tumors. Anti-tumor activity with the TDO2 inhibitor was equally observed in the P815 control implanted tumors negative for TDO2, thus providing evidence for an effect of TDO2 expressed in the immune system of the animal. These experiments for the first time provided clear evidence for a role of TDO2 in regulating tumor growth through expression in the cancer cell as well as immune compartment.
In line with its expression profile in liver, TDO2 was found predominantly in hepatocellular carcinoma (HCC) (Pilotte L et al., Proc Natl Acad Sci USA, 2012, 109(7), 2497-502). Inhibition of tryptophan catabolism and thus restoration of tryptophan concentration and decreased production of downstream metabolites could prove beneficial in the context of liver disease progressing to the stage of liver carcinoma. More particularly: (i) several reports have shown evidence that increased availability of tryptophan through supplementation is beneficial for example, cirrhotic livers, allowing the direct use of tryptophan for protein synthesis (Ohta et al., Amino Acids, 1996, 10(4), 369-78); (ii) there is a correlation between increased downstream serum tryptophan metabolites, such as quinolinic acid, and hepatic dysfunction in patients with liver cirrhosis (Landou et al., Hum Immunol, 2013, 74(1), 60-6) and (iii) increased secretion of another tryptophan metabolite, indole-3-lactic acid, has been associated with alcohol-induced liver disease in mice (Manna et al., J Proteome Res, 2011, 10(9), 4120-33). In the context of liver carcinoma itself, very high RNA expression is a good indication for therapeutic evaluation of TDO2 inhibitors (Pilotte L et al., Proc Natl Acad Sci USA, 2012, 109(7), 2497-502). The above thus provides a clear rationale for TDO2 activity modulation in the control of liver tumor development.
In addition to expression in liver, TDO2 is expressed in neurons, microglia and astrocytes and the potential benefit of TDO2 inhibition in the context of glioma was shown in another animal model. Platten and collaborators demonstrated that the tryptophan catabolite kynurenine produced by TDO expressed in the tumor cells suppresses antitumour immune responses and promotes tumor-cell survival and motility through the AHR in an autocrine/paracrine fashion (Opitz C A et al., Nature, 2011, 478(7368), 197-203).
The TDO-AHR pathway is active in human brain tumors and is associated with malignant progression and poor survival. Further evidence came from the accumulation of a downstream metabolite, quinolinic acid which accumulates in human gliomas and was associated with a malignant phenotype (Sahm et al., Cancer Res, 2013, 73(11), 3225-34). Here tryptophan catabolism was shown to occur in microglia cells as well. The above data thus provides evidence for TDO2 targeting in glioma with brain-penetrant small molecules.
Other tumor types in which TDO2 mRNA was found are breast carcinoma, bladder, renal cell, pancreatic, colorectal, head & neck carcinoma and lung carcinoma as well as melanoma thus broadening the scope of TDO2 targeting beyond HCC and glioma (Pilotte L et al., Proc Natl Acad Sci USA, 2012, 109(7), 2497-502).
The enhanced Tryptophan degradation observed in patients with gynecological cancers (ovarian carcinoma, cervical cancer, endometrial cancer) provides additional rationale for TDO2 targeting in those cancers (Sperner-Unterweger B et al, Immunology, 2011, 216 (3); 296-301).
The tryptophan catabolism in some cancers might be also increased by the expression of indoleamine 2,3-dioxygenase (IDO1) by tumor cells (Uyttenhove, C. et al., Nat. Med., 2003, 9, 1269-1274).
Because tryptophan catabolism is induced by inflammatory mediators, notably IFN-gamma, it is thought to represent an endogenous mechanism that restricts excessive immune responses, thereby preventing immunopathology. However in the context of cancer, there is strong evidence that suppression of antitumor immune responses in precancerous lesions and established cancers by tryptophan catabolism promotes tumor growth, which would make such catabolism an attractive target for therapeutic intervention (Dolu{hacek over (s)}ić E and Frederick R., Expert Opin Ther Pat., 2013, 23(10), 1367-81). Hence, a considerable effort is being made to identify selective and efficient inhibitors of tryptophan catabolism to enhance the efficacy of conventional chemotherapy, immune checkpoints (Holmgaard R B et al., J Exp Med., 2013, 210(7), 1389-402) or therapeutic vaccines.
In the context of neurological brain disorders, TDO2 expression has been demonstrated in neurons, brain vasculature and additionally in the case of schizophrenia in astroglial cells (Miller C et al., 2004, Neurobiology Dis, 15(3):618-29). The kynurenine pathway is now considered as a therapeutic target in cognitive diseases like bipolar disorder or Tourette syndrome and neurodegenerative disorders like Alzheimer, motor neuron disease like Amyotrophic lateral sclerosis, Multiple sclerosis, Huntington or Parkinson's disease (Stone T W, 2013, Br J of Pharmacol, 169(6): 1211-27; Wu et al, 2013, Plos One, 8(4):e59749; Fuvesi et al, 2012, J Neural Transm, 119(2):225-34, Widner et al, 2002, J Neural Transm, 109(2):181-9; Comings et al, 1996, Pharmacogenetics, 6(4):307-18, Forrest 2010, J Neurochem, 112(1):112-22).
Cognitive changes related to Tryptophan catabolism have also been shown in patients infected with human immunodeficiency virus type-1 (HIV), called HIV-associated neurocognitive disorder (HAND) (Davies et al, 2010, Int J of Tryptophan Res, 3:121-40). In addition, T cell hyporesponsiveness has been recently associated with the Tryptophan catabolic pathway in HIV-infected patients with possibly extension to other chronic infectious diseases like e.g. Hepatitis C.
Some TDO2 inhibitors were proposed in WO2010/008427 and by Dolusic, E. et al. (Dolusic et al., J. Med. Chem., 2011, 54, 5320-5334), however either their affinity for the target is limited, or their pharmacokinetic properties are not suitable for development as a drug for human use.
Therefore, there is a need for new TDO2 inhibitors with improved efficacy for cancer treatment and/or prevention.